The modern demands to produce base metals in a profitable yet environmentally safe manner has caused many producers to re-examine their overall processing technology. Copper producers, like all other base metal producers, have long practiced technology that was effective for converting ore to finished products, but this technology must now meet ever-increasingly stringent environmental regulations. To meet these demands, copper producers can either retrofit their existing technology or introduce new copper pyrometallurgy. In either or both cases, the fundamental problem of controlling the destinations of impurities remains.
The pyrometallurgy of copper is a multi-step process, and impurity streams in their various forms are generated at each step. As here used, "impurity" means any component of the starting material that affects the ability to produce pure products and safe disposal streams. In the pyrometallurgy of copper, desired end products other than copper include the valuable metals generally found with copper in the copper ore, e.g., gold, silver, molybdenum, selenium and nickel. Depending upon the copper deposit, other desirable metals may also be present. Typical impurities, other than nonmetallic components such as the silicates and various other gangue components, include antimony, bismuth, arsenic, zinc, cadmium, mercury, iron, tellurium, and the like. While these metal values may be the subject of capture and eventual sale depending upon their concentration in the ore and the form in which they are present in the ore, these metals are typically the subject of capture for ultimate nonsale disposal.
The pyrometallurgy of copper begins with the smelting of copper concentrate to copper matte. This process generates impurities streams in the form of flue dust and APB (i.e. wet gas scrubbing liquids otherwise known as "acid plant blowdown"). During smelting, a process gas stream is produced into which various impurities are volatilized along with dust, the latter being the result of incomplete smelting of concentrate particles and which includes desirable end products. The impurities must eventually be separated from the desirable end products, and eventually eliminated from the pyrometallurgical circuit.
In addition, the smelting process generates gaseous products which have both heat and sulfur values. These gases are first transferred to a cooler, which may be a waste heat boiler for capture of the latent heat value, or to a water quench system if heat recovery is not required. Cooling of the gas by either of these methods results in the removal of dust that is mostly in the form of unsmelted particles. This material contains relatively small proportions of impurities and is generally recycled to the concentrate smelting process.
After cooling of the gas, a further stage of dry gas cleaning (e.g. a bag house, an electrostatic precipitator, etc.) is normally adopted to remove remaining unsmelted particles and species condensed from volatilized impurities. The impurity content of the product of this cleaning is much higher than the previously cooled material which, in the case of a copper concentrate with a low impurity content, can be returned to the concentrate smelting stage without major impact on the impurity content of the pyrometallurgical products. In the case of a copper concentrate with a high impurity content, this material must be processed separately from the concentrate smelting stage to avoid unduly raising the impurity content in the pyrometallurgical products.
After dry cleaning the gas, it still contains a minor quantity of unsmelted particulate and volatile impurities, the nature and quantity of volatile impurities dependent on the temperature at which the gas was cooled. This gas also contains sulfur dioxide and sulfur trioxide values from the concentrate smelting process, which at certain locations around the world, is presently released to the atmosphere through a stack. However, in an increasingly environmentally conscious world, this practice is becoming less acceptable. Consequently, the sulfur dioxide and trioxide are now routinely the object of capture, and this is commonly accomplished in an acid plant. However before this gas can be used as a feed to an acid plant, it must be further cleaned in a wet scrubbing operation to remove the last traces of unsmelted particulate and volatile impurities.
During this wet scrubbing operation, sulfur trioxide is captured and forms a dilute sulfuric acid solution. At the same time the particulates are captured in solid form as a dilute slurry, and the volatile impurities are condensed to form either a solid material in the dilute slurry or dissolve in the dilute sulfuric acid. This dilute slurry must be removed from the scrubbing system as a bleed stream, and this is known as APB.
The copper matte produced by the smelting process is an intermediate which is then converted to blister copper in a conversion process. This process also generates a process gas containing unconverted particles and volatilized impurities both of which are treated in a manner similar to the concentrate smelting process gas.
In addition to the impurity streams generated in the smelter and converter, impurity streams are also generated in the fire refining of blister copper to anode copper in the anode furnaces, and in the associated processes of electrorefining of the anode copper and precious metal refining. The impurity streams from the electrorefining of copper and from the refining of precious metals can be variously liquids, solids and slimes.
Due to the presence of desirable end products, e.g., principally copper, gold and silver, and the desire to minimize the ultimate amount of material that must be removed from the pyrometallurgical process for nonsale disposal, these impurity streams are recycled to the fullest extent possible. However, recycle, if not carefully controlled, will inevitably result in increasing the amount of impurities in the intermediate and final products to a point at which the products are unacceptable. Different impurities have different impacts on the pyrometallurgical process and the properties of its end products.
For example, bismuth is known to be an embrittling agent in copper cathode (the desired copper end product) and although its general specification calls for less than 1 ppm, its use for wire drawing demands levels as low as 0.25 ppm. Unfortunately, bismuth has a great affinity for the copper phases of copper smelting and as such, a relatively small amount in the process, be it from the original copper concentrates or recycled impurity streams, can have a relatively large affect on copper cathode quality. Similarly, tellurium and selenium are also embrittling agents but since neither are very soluble in copper electrolyte, neither transfer to the copper cathode in any appreciable amount.
Antimony is similar in its behavior and concentrations in the overall process to that of bismuth although it impacts the quality of copper cathode differently. Whether antimony reaches the maximum level in cathode copper before bismuth does so is dependent upon the relative proportions of these elements in copper concentrate, and also upon their deportments in the particular copper smelting technology in use.
Although the target amount of lead in cathode copper is also relatively small, e.g. less than 5 ppm, the amount of lead in the overall process can be, and often is, orders of magnitude larger than that of bismuth and antimony. However unlike bismuth and antimony, a certain level of lead in anode copper (the penultimate copper end product) is beneficial to the production of copper cathode because lead contributes to the rejection of bismuth and antimony into the anode slimes (and thus obstructs their dissolution into copper electrolyte and their consequent deposition into the cathode copper).
Arsenic can have levels in the concentrates and the process intermediate products similar to lead, and it too has a beneficial level in anode copper. General refining practice is to require levels of arsenic equal to or greater than three times the combined molar composition of bismuth and antimony in the anode copper. This is believed to promote, in conjunction with the presence of lead, the deposition of bismuth and antimony into anode slimes. In some cases, smelters may add purchased arsenic in various forms to the smelting process to optimize the arsenic level in anode copper. Moreover, certain levels of arsenic in copper electrolyte also have a promotional effect in deporting bismuth and antimony to anode slimes, and thus inhibiting their dissolution in the electrolyte and possible ultimate deposition into the copper cathode.
Cadmium is an impurity that is found in close association with zinc but at much lower concentrations. While zinc deports in large measure to the smelter slag, cadmium preferentially volatilizes into the process gas and deports to dry dust and APB. While cadmium does not finally deport to the copper cathode, if repeatedly recycled to the smelter, it will build in concentration to a point that it becomes a health and environmental issue.
Like cadmium, mercury forms volatile species that report to the gas stream and in this case, almost exclusively to APB. If allowed to build in concentration in the gas stream, then it can have an adverse impact on the quality of the sulfuric acid produced by the plant.
The traditional methods of controlling these and other impurities have been the separate or partially integrated processing of the APB, the smelting and converting dusts, and the copper and precious metal refining bleeds. In some cases, these impurity streams are processed to remove at least a portion of the impurities present in the stream before the stream is recycled back to the pyrometallurgical circuit. In other cases, a portion of the impurity stream itself is simply removed from the circuit, e.g. flue dust can be collected and sold to various processors as a feed material for their operations, e.g., lead-zinc smelting facilities. However, due to the presence of valuable primary product in these impurity streams, such practices are often economically undesirable and in some remote locations, simply not available.
Over the years, copper processors have developed and operated various hydrometallurgical processes for treating flue dust for capture of its valuable components and for the ultimate removal of its undesirable components from the pyrometallurgical circuit. These hydrometallurgical processes have taken various forms, but usually involve the acid leaching of the dust to solubilize the metal components, and then the sequential precipitation of these various components. If possible, precipitated material, e.g., copper sulfide or hydroxide, is recycled to the smelter, and where not possible or practical, precipitated material, e.g., ferric arsenate or arsenic sulfide, is rendered environmentally acceptable and disposed, e.g., to a tailings pond or a managed hazardous materials facility. While most of these processes have proven effective in one manner or another, all are subject to improvement, particularly with respect to increased capture of desirable end products, energy reduction, and the reduction of materials ultimately discharged to the environment.